The present invention relates to a system for making an electrical connection between an electrochemical cell and the copper track of a printed circuit. The invention also extends to a method of making such a connection.
Electrical current is input or output relative to an electrochemical cell by means of terminals that extend from the cell and that are respectively of negative and positive polarity. When cells are assembled together to form a battery, they are electrically interconnected in series or in parallel. Connections between cells are usually made by means of an electronics card carrying an interconnection printed circuit. The card is generally designed to enable an electronic safety device to be inserted in the battery.
A connection device comprising one or more electrically interconnected pieces is welded to the terminal of the electrochemical cell and its opposite end is soldered to the copper layer constituting the track of the printed circuit. The term xe2x80x9csolderingxe2x80x9d is used to designate any operation whereby metal pieces are assembled together by means of a filler metal having a melting point that is lower than that of the pieces to be united. Soldering to the layer of copper is usually performed with the help of flexible conductors.
Making such a connection and feeding it with solder involves drawbacks which can make numerous batteries unusable or even dangerous. In particular, solder can be splashed onto the electronics card during the soldering operation, thereby running the danger of damaging a component or giving rise subsequently to short circuits. In addition, it has been found that short circuits can arise because of subsequent migration of solder.
The use of flexible wires also means that there is a risk of the wires being swapped over during assembly. In addition, using flexible wires makes it difficult to automate manufacturing operations and means that a larger number of inspections must be performed.
An object of the present invention is to propose an electrical connection system between an electrochemical cell and a printed circuit, which system is more reliable than the known system both in terms of manufacturing reject rate and in terms of battery lifetime.
The present invention provides an electrical connection system between an electrochemical cell and a printed circuit having a copper layer, said system comprising at least one connection piece welded to said cell, wherein said layer of copper is of a thickness greater than 70 xcexcm and wherein a metal piece electrically connected to said connection piece is fixed to said layer of copper by welding without any filler metal.
The term xe2x80x9cweldingxe2x80x9d is used to designate an operation consisting in uniting metal pieces in such a manner as to provide continuity, and in particular electrical continuity, between said pieces. In this case welding is performed by applying melting heat and/or pressure, and without using a filler metal.
When the layer of copper is 70 xcexcm thick or less, welding cannot be performed. Under such circumstances, the layer of copper is observed to melt or come unstuck. The layer of copper is preferably at least 105 xcexcm thick.
The thickness of the metal piece is at least 80 xcexcm and preferably lies in the range 80 xcexcm to 200 xcexcm. In a particular embodiment of the invention, said piece is at least 100 xcexcm thick and preferably its thickness lies in the range 125 xcexcm to 140 xcexcm.
The metal piece is a conductive piece made of a metal selected from nickel, nickel-plated steel, stainless steel, and copper.
In a particular embodiment of the invention, said piece constitutes said connection piece itself. However the nickel piece could also be connected to the cell via one or more other pieces, with electrical continuity being provided by welding said pieces together. In particular, the metal piece can be welded to the connection piece which is itself made of nickel, of steel, or of nickel-plated steel. Thus, electrical continuity between the cell and the interconnection circuit is provided solely via welds that do not include any filler metal.
The present invention has the advantage of making it simpler, more reliable, and less expensive to interconnect electrochemical cells in a battery, specifically by reducing welding time by about 60% and dividing the reject rate due to assembly errors or to damage to electronic circuits by 10. In addition, assembly operations are easier to automate.
The resulting weld can be characterized by the usual methods, mainly its electrical properties, by measuring its resistance, and its mechanical properties by measuring its breaking strength when subjected to a perpendicular traction force or by measuring its shear strength under an axial traction force.
The resistance of the connection after said piece has been welded to said copper layer is no greater than 0.07 mxcexa9 for a current of 5 A.
The breaking strength after said piece has been welded to said layer of copper is not less than 1.5 daN, and the shear strength is not less than 8 daN.
The invention also provides an electrochemical cell connected to a printed circuit by such an electrical connection system, and a battery of such electrochemical cells. The battery can equally well be of primary cells or of secondary cells. For example, the invention applies to cells having an organic electrolyte such as LiSO2 primary cells or lithium-ion secondary cells, or indeed to cells having an aqueous electrolyte such as nickel-cadmium secondary cells or nickel-metalhydride secondary cells. The cells can be cylindrical or prismatic in shape. The number of cells making up the battery is not limited. The cells can be connected together in series and/or in parallel.
The invention also provides a method of manufacturing a system as defined above, in which said piece is resistance welded to said layer of copper. The resistance welding is preferably spot welding.
Spot welding is performed using electrodes of cylindrical shape having a diameter of about 1.2 mm. The electrodes are preferably made of copper or copper alloy.
The welding is performed at electrical energy lying in the range 50 W.s to 100 W.s. The total time required to perform said welding is much less than 1 second.
Welding is performed at a pressure lying in the range 60 N/mm2 to 80 N/mm2.